Allow setting facility on SyslogWriter

[tchernobog]

To allow logging with different facilities, we need to be able to set the facility on the SyslogWriter struct. This would allow to use something like:

log::info(target: "auth", "Message for audit log");

assuming the right layer is setup with tracing.

Please add a method to set the facility on the writer, e.g.

impl SyslogWriter {
    /// Set the facility this writer is using.
    pub fn set_facility(&mut self, facility: Facility) {
        self.facility = facility;
    }
}